1. Field of the Invention
The present invention relates to a bias circuit, and more particularly, to a bias circuit having a bandgap reference.
2. Description of Related Art
To stabilize power supply variation characteristics of a semiconductor integrated circuit, it is necessary to provide a constant current bias and a low temperature coefficient reference voltage which are not affected by temperature in the integrated circuit. For this reason, there has heretofore been widely used a bias circuit having a function of keeping a circuit current substantially constant independently of a power supply applied voltage, by use of a constant current source employing a bandgap reference (hereinafter, referred to as “BGR”). Further, as a bias circuit of this type, there is used a bias circuit having a current supply path for the BGR to be ready for a stable operation in a short period of time from a rise time during power-on.
FIG. 4 shows a configuration example of a bias circuit 40 of a related art, and FIG. 5 shows an operation waveform of the bias circuit 40. The bias circuit 40 includes a BGR 102 and a current path 100. The BGR 102 supplies a bias to an internal circuit of a semiconductor integrated circuit. The current path 100 supplies a current for causing the BGR 102 to operate. The current path 100 includes devices 105 and 106 and a resistor element R101. The device 106 (corresponding to PMOS transistor Tr106 in this case) is current mirror connected to a transistor provided in the internal circuit of the semiconductor integrated circuit. The device 105 (corresponding to bipolar NPN transistor Tr105 in this case) is connected in parallel with the resistor element R101.
As a power supply voltage, which is an output voltage from a power supply voltage terminal 107, gradually increases from 0 V during a time period ton_d, a current I101, which is supplied to the BGR 102 from the PMOS transistor Tr106 through the resistor element R101 in the current path 100, also increases. Then, the current I101 flows in an amount necessary and sufficient for starting the BGR 102, and the BGR 102 is started at a timing ton_s. The BGR 102 performs a constant current operation, so a transistor device 103 (corresponding to bipolar NPN transistor Tr103 in this case), which is current mirror connected to the BGR 102, also performs the constant current operation. As a result, a constant voltage generation circuit (hereinafter, referred as “VREG”) 104, which is connected between the power supply voltage terminal 107 and a collector of the transistor Tr103, operates. Accordingly, a constant voltage is output to a node 109, thereby turning on the transistor Tr105.
From that time, most of the current supplied to the BGR 102 is supplied as an emitter current I105 of the transistor Tr105, and a voltage at a node 108, which is a current supply point for the BGR 102, is also stabilized by a constant voltage output of the VREG 104. As a result, the BGR 102 is not affected by a voltage fluctuation during a time period for the BGR 102 to reach a final voltage (30 V, for example) from a start time in a time period ton_cons, and operates as a constant current consumption circuit. Therefore, a current I106 flowing through the transistor Tr106 is kept constant, thereby enabling a constant current bias operation for the internal circuit of the semiconductor integrated circuit.
However, in the bias circuit of the related art, during the time period ton_cons, as the power supply voltage increases, a voltage across both ends of the resistor element R101 also increases. As a result, the current I101 flowing through the resistor element R101 also increases. Accordingly, contrary to the increase in current with an increase of the power supply voltage, the current I105 flowing through the transistor Tr105 decreases. Further, when the power supply voltage exceeds the final voltage (30 V, for example) and further increases (to 40 V, for example), the current I105 becomes 0 A at a time ton_f, and the transistor Tr105 is turned off, whereby the node 108, which is kept at the constant voltage during the time period ton_cons, becomes incapable of performing the constant voltage operation. Accordingly, after the time ton_f, the current of the resistor element R101 continues to increase and the current supplied to the BGR 102 also increases. In other words, when the node 108 is not kept at the constant voltage, the BGR 102 shifts the operation from the constant current operation to a fluctuation operation, with the result that the bias circuit 40 itself becomes unstable due to the fluctuation of the power supply.
To solve the above-mentioned problems, there can be employed a method in which a resistance value of the resistor element R101 is set as large as possible, and an amount of a current (and current change amount) flowing through the resistor element R101 with an increase of the power supply voltage, is reduced, to thereby make the time period ton_cons longer. However, in this case, a current supply period (time period ton_d) for starting the BGR 102 also becomes longer, whereby it takes long time to start the operation for stabilizing the bias circuit 40. In other words, the stable operation of the bias circuit in a wide range of an applied voltage of the power supply voltage is incompatible with the reduction in time for stabilization during the starting operation.
As described above, in the related art, with the increase of the power supply voltage, the bias circuit becomes unstable for a long time period time due to the fluctuation of the power supply in some cases.